[Non-BE] Resolve "ISM mono downmix accumulating energies"

• Related issues: #967 (closed)
• Requested reviewers:

Reason why this change is needed

• The function ivas_mono_downmix_render_passive() uses an equalization factor that is calculated from two energy values. These energy values accumulate over time and create very big numbers which are likely to cause issues in a fixed-point implementation. A re-investigation is thus needed.

Description of the change

• ivas_mono_downmix_render_passive() is adjusted to do the equalization factor calculation in a frame-by-frame manner by introducing local variables and doing auto-regressive smoothing. With the added (1-DOWNMIX_ALPHA) term, the energie values don't increase unreasonably.
• The function is further renamed to ivas_ism_mono_dmx() as it is not really a passive downmix.

Affected operating points

• Mono output for modes ISM (Discrete and Parametric), OMASA, OSBA
• Encoder unchanged, bitstream BE
• Decoder affected for rendering to mono, output non-BE

Related to #967 (closed)